Because a silicon carbide (SiC) semiconductor is higher in dielectric breakdown field, saturated drift velocity of electrons, and thermal conductivity than a silicon semiconductor, researches and developments have been made on the use of the silicon carbide semiconductor for realizing a power device capable of operating at higher temperatures and at a higher speed than conventional silicon devices. The development of a highly efficient switching element used in power sources for driving motors of electric bicycles, electric vehicles, hybrid cars and the like has been drawing attention. In order to realize such a power device, a silicon carbide single-crystal substrate having a smooth surface is required as a substrate where to epitaxially grow a high-quality silicon carbide semiconductor layer.
Further, as a light source for high-density information recording, a blue laser diode has been drawing attention, and a need for a white diode as a light source to replace fluorescent lamps and bulbs is also increasing. A gallium nitride (GaN) semiconductor is used to fabricate such a light-emitting element, and as a substrate where to form a high-quality gallium nitride semiconductor layer, a silicon carbide single-crystal substrate is used.
For the silicon carbide single-crystal substrate for such applications, high machining accuracy is required in terms of flatness of the substrate, smoothness of the surface of the substrate, and the like. However, because a silicon carbide single crystal has very high hardness and excellent corrosion resistance, its workability when the substrate is fabricated is poor, which makes it difficult to obtain the silicon carbide single-crystal substrate having high smoothness.
Generally, a smooth surface of a semiconductor single-crystal substrate is formed by polishing. When a silicon carbide single crystal is polished, abrasive grains of diamond or the like harder than silicon carbide are used to mechanically polish the surface to form the flat surface, but minute scratches according to a particle size of the diamond abrasive grains are introduced in the surface of the silicon carbide single-crystal substrate polished by the diamond abrasive grains. Further, since a work-affected layer having a mechanical strain is generated in the surface, the surface of the silicon carbide single-crystal substrate does not have sufficient smoothness as it is.
In the manufacture of the semiconductor single-crystal substrate, as a method for smoothing the surface of the semiconductor substrate having undergone the mechanical polishing, chemical mechanical polishing (sometimes hereinafter referred to as CMP) technology is used. The CMP is a method to use a chemical reaction such as oxidation to change a workpiece to an oxide or the like and use abrasive grains lower in hardness than the workpiece to remove the generated oxide, thereby polishing its surface. This method has an advantage of being capable of forming a very smooth surface without causing a strain in the surface of the workpiece.
As a polishing agent for more smoothly polishing the surface of the silicon carbide single-crystal substrate by the CMP, a polishing composition with pH4 to 9 containing colloidal silica has been conventionally known (refer to JP-A 2005-117027 (KOKAI), for example). There has also been proposed a polishing composition containing silica abrasive grains, an oxidant such as hydrogen peroxide (oxygen imparting agent), and vanadate (refer to JP-A 2008-179655 (KOKAI), for example). Furthermore, there has also been proposed a polishing composition containing abrasive grains of alumina, titania, ceria, zirconia, or the like, an oxidant such as hydrogen peroxide, permanganate, or periodate, and a dispersion medium (refer to WO 2009/111001 A1, for example).
However, in the use of the polishing composition described in JP-A 2005-117027 (KOKAI), a polishing rate for the silicon carbide single-crystal substrate is low and thus it takes a very long time for the polishing. Further, the use of the polishing composition described in JP-A 2008-179655 (KOKAI) has a problem that a polishing rate is not high enough and thus it takes a long time for the polishing. Further, the use of the polishing composition described in WO 2009/111001 A1 also has a problem that aggregation of the abrasive grains occurs in a short time and it is not possible to obtain a sufficient polishing rate.
There has been proposed a method of smoothly polishing a surface of a silicon carbide single-crystal substrate or the like in the presence of an oxidative polishing liquid by using a polishing pad including abrasive grains (refer to JP-A 2008-068390 (KOKAI), for example). However, in the method described in JP-A 2008-068390 (KOKAI), a polishing rate is not high enough and the surface suffers damage such as a scratch or a strain due to a strong mechanical action of the fixed abrasive grains. Further, it is difficult to adjust a particle size, the content, and content distribution of the abrasive grains included in the polishing pad and is difficult to obtain a smooth surface after the polishing.